Hydrocarbon recovery operations may in general involve a broad range of processes involving the use and control of fluid flow operations for the recovery of hydrocarbon from subterranean formations, including for instance the inserting or injection of fluids into subterranean formations such as treatment fluids, consolidation fluids, or hydraulic fracturing fluids, water flooding operations, drilling operations, cleaning operations of flow lines and well bores, and cementing operations in well bores.
Employing pressure pulse technology (PPT) in hydrocarbon recovery operations has gained significant interest during the last few years and there are many patent application and patents where PPT is included.
Hydrocarbon recovery operations may for instance require tools for cleaning of casing, deposits from near well bore areas, perforations and screens. In wells with increased water production (waterflood projects) and geothermal wells, scale and deposit buildups are often a major cause of decreased production. Conventional methods of removing such buildups such as acid wash, wire line broaching and even replacing the production string and flow lines are often either expensive or provide only limited success. A further method to clean fluid flow channels or well bores involve the application of pulsating fluid flow as disclosed in e.g. WO2009/063162 and WO2005/093264 where the use of a pulsating fluid flow for the cleaning of surfaces is described as advantageous in comparison to steady fluid flow.
Another hydrocarbon recovery operation where the application of pressure pulses has been described comprises the chemical insertion into a well bore matrix or insertion of treatment fluids into a subterranean formation. The effectiveness of such methods depend among other things on the ability of the insertion fluid to penetrate the formation which often comprises shales, clays, and/or coal beds of generally a low permeability.
Further, wells are often located in unconsolidated portions of a subterranean formation that contain particles capable of migrating with the flow of a mixture of hydrocarbons and fluids out of a formation and into a well bore. The presence of these particles, such as sand, is undesirable since they may destroy pumps and other producing equipment. One conventional method is to apply a resin composition to the unconsolidated area and then to after-flush the area with a fluid to remove excess resin from the pore spaces of the zones. Such resin consolidation methods are widely used but are limited by the ability of the consolidation fluid (often a resin composition) to achieve a significant penetration or uniform penetration into the unconsolidated portions of a subterranean formation. Methods for injecting a consolidation fluid into a wellbore, as disclosed in US2009/0178801, describes the use of pressure pulsing to enhance the ability of a consolidation fluid to penetrate a portion of a subterranean formation.
In cementing operations in well bores, cement is typically pumped into an annulus between the wall of a well bore and the casing disposed therein. The cement cures in the annulus and thus forms a hardened sheath of cement that supports the pipe string in the well bore. Influx of fluid and gas during the cement curing is common, and this can damage the cement bond between the well bore formation and the exterior surface of the casing. Methods for reducing fluid or gas migration into the cement are disclosed in e.g. US2009/0159282, comprising the step of inducing pressure pulses in the cement before the cement has cured.
The injection of hydraulic fracturing fluids into subterranean reservoir formations makes it possible to produce hydrocarbons where conventional technologies are ineffective, and the method applies fluid pressure to create fracture in the subterranean reservoir formation allowing hydrocarbons to escape and flow out of a well. Today, through the use of hydraulic fracturing, large amount of deep shale natural gas from across the United States are being produced. Applying pressure pulses during the hydraulic fracturing process has been suggested in order to increase the production of shale natural gas.
Pressure pulse technology may likewise be applied to water flooding operations, where a fluid is continually injected into a subterranean formation while pressure pulses are employed to the fluid as it is being injected.
In general, pressure pulses have been reported to allegedly yields enhancement of flow rates through porous media. However, at present, the literature in the field seem undetermined on the advantages on pulsed injections, as some experiments report on the ability of PPT to increase the recovery factor of hydrocarbons from laboratory core plugs, while some literature report on a lower recovery rate compared to static water flooding. Notice that an increased recovery factor could have many causes, so that a possible effect of pressure pulses alone may be difficult to isolate since the pulsating flow could also contribute.
The enhanced flow rates in porous media allegedly obtained by means of dynamic excitation through applications of pressure pulses has by some been claimed to occur due to the pressure pulses suppressing any tendency for blockage thereby maintaining the reservoir in a superior flowing condition. Also, secondary recovery operations involving replacing a fluid (hydrocarbons) in a porous media (the subterranean reservoir formation) with a second fluid (normally water) is claimed to be enhanced by pressure pulses.
Documents disclosing apparatus for the generation of pressure pulses (sometimes referred to as fluidic oscillators) include e.g. WO2004/113672, WO2005/093264, WO2006/129050, WO2007/100352, WO2009/089622, WO2009/132433, U.S. Pat. No. 6,976,507, and US2009/0107723. Pressure pulses may for instance be generated through a mechanism of convective combustion as described in WO2007/139450, or by igniting a plurality of individual lengths of energetic material as outlined in WO2009/111383 and US2009/0301721.
As mentioned, the application of pressure pulses has been suggested in all the hydrocarbon recovery operations listed above. Further, pressure pulses has likewise been suggested to be used in drilling operations, another hydrocarbon recovery operation. It has also been suggested to apply pressure transients in order to increase the force by which the drill bit is pushed through the subterranean formation as an alternative to using static pressure and drill string weight alone. The pressure transients applied during the drilling operation are conventionally generated by opening and closing valves. Therefore, the flow of drill mud to the drill bit is discontinuous since the flow is interrupted by the closing of the valves.
The amount of hydrocarbon that is recoverable from subterranean reservoirs depends on a number of factors such as the viscosity of the oil, the permeability of the reservoir, and factors like any gas present, pressure from surroundings like adjacent water etc. In general, oil recovery rates employing fluid injection may typically lie in the order of 30-55%, and bearing in mind the impressive potential extra profit obtainable from even very small increases in the oil recovery rate, the presently applied methods in hydrocarbon operations leave ample room for improvements.
As noted above, the use of pressure pulse technology in hydrocarbon recovery operations has gained increasing interest in recent years. More generally, pressure may be formed and applied in different ways, which in view of the proposed methods according to the present invention and the terms used herein, is explained in more detail in the following.
On a microscopic level pressure is the results of the thermal motion of the particles in the fluid, and one can interpret pressure as energy density in the fluid. However, on a macroscopic level pressure is more commonly regarded as the ability of the fluid to exert a force on a body. The force F that the pressure inside a hydraulic cylinder can exert on a piston is given by F=Ap, in which A is the size of the surface of the piston which is in contact with fluid inside the hydraulic cylinder. Hence, a standard method of producing a pressure p inside a hydraulic cylinder is to apply a force F on the piston, thereby obtaining a pressure given by p=F/A. In this way a static pressure can be generated by a constant force.
A pressure wave is an oscillation of the pressure amplitude in time and space with a given maximum amplitude and frequency. A standing pressure wave has only a variation in time with a frequency equal to the resonant frequency of the system. The standard method of obtaining such pressure waves are by employing an oscillating piston in the fluid, which is thus moved with a given frequency and amplitude.
Pressure pulses can be generated with a piston moved sufficiently fast, but in this case there is not necessarily a given frequency for the motion of the piston. Such an impulse piston could be constructed by use of materials that change their shape in the presence of magnetic fields as explained in US2009/0272555. Typically, the piston is moved fast forward producing the pressure pulse, with a subsequent relatively slow movement backwards. The motion of the piston need not be periodic, and the word frequency does not really have any meaning when describing a pressure pulse. However, the term “frequency” may often be applied in order to specify the time interval between each pressure pulse if generated at regular intervals. An example of such pressure pulse generation is disclosed in WO2004/113672 where a piston is forced up and down within a cylinder by a power pack assembly. The use of such impulse piston however yields a significant increase in the flow rate during the fast movement of the piston and thus during the generation of the pressure pulse.
Pressure pulses may similarly be produced by employing a pressure chamber, where the pressure pulse may be generated in a fluid outside of a pressurized chamber when a valve at the outlet of this chamber is opened rapidly. The outlet valve is then closed and the chamber is filled and pressurized once more by a pump pushing fluid into the chamber through the chamber inlet. The cycle is then repeated in order to generate pressure pulses with a fixed or arbitrary time interval. The term “pressure pulse” originates from this method since a pump and a pressure chamber is needed, which can be associated with the human heart where one chamber then functions as a pump and the other as a pressure chamber.
Applying this last procedure for generating pressure pulses also results in a discontinuous fluid flow since the closing of the valve interrupts the fluid flow.
In general a pressure pulse can be said to have many of the properties of a pressure wave, such as moving with the speed of sound throughout the fluid, and being reflected and transmitted much like a wave. The main difference between pressure pulses and pressure waves is, that pressure pulses in general have a shorter rise time and slow decay rate, i.e. they do not possess the typical periodic sinusoidal shape which is characteristic for pressure waves. Pressure pulses propagate like relatively steep fronts throughout the fluid in comparison to pressure waves moving with a sinusoidal profile. Supposedly the steep front or the relatively short rise time makes the pressure pulses advantageous for applications in hydrocarbon recovery operations.
Understanding the term pressure transients as applied herein and the procedure for generating said pressure transients is important in order to understand the underlying concept of the method described in this disclosure.
An important difference between pressure pulses and pressure transients is related to the two most fundamental laws in nature; conservation of energy and momentum. One may say that pressure pulses do not contain momentum, whereas pressure transients do contain momentum. In fact, momentum is converted into pressure transients during a collision process as will be explained in more details in the following.
There are many methods that can be applied in order to produce a pressure pulse, but to our knowledge there is only one procedure for generating a pressure transient, namely by performing a collision process. Pressure transients in fluids occur in two different types of collisions; 1) when a solid object in motion collides with the fluid, or 2) when a flowing fluid collides with a solid. In the first case, momentum of the solid object is converted into pressure transients in the fluid via the collision process. The last case describes the Water Hammer phenomenon where momentum of the flowing fluid is converted into pressure transients in the fluid. In both cases pressure transients are produced in the fluid.
In a collision process the immense impacting force on the body and resulting loads on the fluid are of large magnitude and short duration so that the dominant terms in describing the motion of the fluid reduce to conservation of momentum. Further, the time scales are so short that the convective terms in the fluids acceleration are negligible. The collision process therefore result in a travelling pressure transient of very high amplitude of a very small duration and of a very steep front compared to conventional pressure pulses.
The conversion of momentum into pressure transients can be explained in more detail by analysing the Water Hammer phenomena where a fluid flowing in a pipeline (with cross section σ) is forced to stop during a time interval Δt due to a sudden closure of a valve. To solve this problem one can follow the work by N. Joukowsky. Newton's second law can be written in the momentum form FΔt=Δ(mu), where F is the force, Δt is a time interval and Δ(mu) is the change in momentum of a body with mass m and velocity u. By applying that a pressure transient can be expressed as Γ=F/σ one thus obtains ΓσΔt=ρuV=ρuσL=ρuσcΔt, where σ is the cross section of the pipeline, Δt is the time interval of the momentum change Δ(mu), V=σL is the volume V of the part of the fluid (with density ρ) that has lost its momentum, and L is the length that the pressure transient Γ has propagated with the sound speed c during the time interval Δt. The well-known Joukowsky equation Γ=ρcu is thus obtained.
Joukowsky by the work outlined above, has demonstrated that momentum of a flowing fluid can be lost if said momentum is converted into pressure transients in the fluid. Hence, Joukowsky has explained the paradox that momentum of a flowing fluid has been lost during the Water Hammer phenomena. The paradox is related to the fact that momentum must always be conserved, but Joukowsky solved this paradox by showing that pressure transients are produced. Hence, momentum is conserved only if said pressure transients contain said momentum.
This applies also for a moving solid object and not only for a flowing fluid. Notice also that the reversed phenomenon is also true. Pressure transients can only disappear if converted into momentum of a moving solid object or a flowing fluid. Momentum is commonly acknowledged as an important physical property which is usually assumed to only be present in moving solids or flowing fluids. However, Joukowsky has demonstrated that momentum is also contained in pressure transients, but in this case said momentum is not a fluid motion or a motion of a solid object. Pressure transients do not represent any material (atoms or molecules) motion, nevertheless they contain momentum.
This property of the pressure transients induced by a collision process may be advantageous when it comes to mobilizing hydrocarbons that normally are immobile when other prior art methods are applied. This property is something that pressure pulses are lacking. Pressure pulses do not contain momentum, which is in contrast to pressure transients that are compelled to conserve the momentum of the object employed in the collision process that created said pressure transients. This property further makes it possible to claim that pressure transients behave as particles.
In summary, pressure transients can be produced by use of a piston, where a moving solid object collides with the piston (body). Hence, pressure transients can also appear in a fluid if a solid object collides indirectly through another body (such as a piston) with a fluid.
Pressure transients (also often referred to as pressure surge or hydraulic shock) have primarily been reported on and analysed in relation to their potentially damaging or even catastrophic effects when unintentionally occurring for instance in pipe systems or in relation to dams or off-shore constructions due to the sea-water slamming or wave breaking on platforms. Water Hammering may often occur when the fluid in motion is forced to stop or suddenly change direction for instance caused by a sudden closure of a valve in a pipe system. In pipe systems Water Hammering may result in problems from noise and vibration to breakage and pipe collapse. In order to avoid Water Hammering pipe systems are most often equipped with accumulators, bypasses, shock absorbers or the like. One reason for the damaging effects caused by the Water Hammer phenomenon is the formation of cavitations in the fluid system. Such cavitations may occur as the pressure transients in a closed system are prevented from being converted back into momentum and instead are converted into cavitations.
As mentioned, pressure transients may be achieved by the so-called Water Hammer effect as e.g. described in WO2009/082453. The methods described therein involve drilling operations where the flow of the drilling fluid is interrupted by a valve, and the repetitively cycle of opening and closing of the valve generates pressure transients that propagate towards the drill bit with the purpose of enhancing the rate of penetration of the drilling operation. The pressure transients are allegedly pushing the drill bit through the subterranean formation with a substantially higher force than would be achieved using pump pressure and drill string weight alone. Further, employing the Water Hammer effect and the thereby generated pressure transients allegedly has a positive effect on rock chip removal and drilling penetration rate. Examples of such devices exploiting the Water Hammer effect may be found in e.g. U.S. Pat. No. 4,901,290, U.S. Pat. No. 6,237,701, U.S. Pat. No. 6,910,542, U.S. Pat. No. 7,464,772, WO2005/079224, and WO2009/082453. Common to these devices is that the pressure transients are created by the rapid closing and opening of valves, which however is disadvantageous in resulting in a discontinuous fluid flow. Further, the size and thereby the propagation of the pressure transients generated by such opening and closing may be difficult to control.
Another apparatus for generating pressure transients is described in WO2010/137991 for the use in transporting and pumping of fluids. This apparatus generates the pressure transients by employing an object with nonzero momentum which is colliding with a body.
As mentioned above pressure pulses propagate like a relatively sharp front throughout the fluid in comparison to a pressure wave. When comparing pressure transients to pressure pulses, one notice that pressure transients have an even sharper front and travels like a shock front in the fluid as is observed during the Water Hammer phenomena. Pressure transients therefore exhibit the same important characteristic as pressure pulses, but they possess considerably more of this vital effect of having a sharp front or a short rise time. The amplitude of the pressure transients which may be obtained, depend on the initial momentum of the colliding objects (i.e. the masses and initial velocities of the objects involved in the collision process) and on the compressibility of the fluid. An example of this is given in the FIG. 6B, where a pressure transient with amplitude of about 170 Bar (about 2500 psi) has a duration of about 5 ms at the point of measure. This gives an extremely short rise time of about 35 000 Bar/sec for the pressure.
In comparison, during the generation of pressure pulses in a fluid where no momentum is converted from any impacting object, a considerable amount of the energy is applied to move the pulse aggregate (such as the strokes of a piston) and thereby pure transport of the fluid. This is not advantageous since the pressure pulsing device is normally intended to be employed together with a fluid injection device which is more efficient when it comes to transporting fluids.
The particle behaviour of pressure transients may be illustrated by observing the Newton cradle (a popular classic desk toy), where the impact of a first ball from the one side sets the outermost last ball at the opposite side in motion with almost no motion of the balls in between. The momentum of the first ball is converted into a pressure transient that travel trough the intermediate balls, and when the pressure transient arrives at the last ball it behaves as a particle setting this ball in motion. In this way, the momentum from the first ball has been converted into a pressure transient that propagates through the balls in the middle and it is finally converted into momentum, and thus motion, of the outermost last ball. This illustrates the temporary nature of pressure transients. Notice also that the pressure transient has also conserved the energy, thus the conservation of both these laws give the peculiar effect that the impact of two balls at the left result in a corresponding motion of two balls at the right and this applies for any number of balls.
One should realize that, contrary to common belief, the conservation laws of energy and momentum alone are not sufficient to explain this behaviour completely, and a further condition must be satisfied by the systems of balls in the Newton cradle. Said system must be capable of a close to dispersion-free energy propagation. Thus, the pressure transients must propagate with almost no energy losses as described in e.g. Am. J. Phys. 49, 761 (1981) and Am. J. Phys. 50, 977 (1982). This effect can be important when employing pressure transients in hydrocarbon recovery operations.
Pressure transients may be seen as an entity in a temporary or transitory state due to the fact that pressure transients are compelled to conserve the momentum of the object employed in the collision process creating the pressure transients. A pressure transient, which propagates in a fluid, is a temporary state which eventually is converted into a motion of the fluid and/or some object in contact with fluid. Ignoring any energy losses during the process, the final motion should ideally yield a total momentum equal to the momentum initially lost by the first object applied in the collision process where the pressure transients were generated.
In comparison, pressure pulses and pressure waves do not possess any temporary nature as described above in relation to pressure transients, in that pressure pulses and waves may dampen out as they propagate in a fluid due to dissipation effect, but they cannot disappear in the same way as pressure transients when eventually converted back into momentum.